Sony has officially confirmed that the PlayStation 4 will be able to
play second-hand games after all, after a patent on a method of blocking
them and the much enhanced online delivery system of the new console led to much speculation that it would finally axe the used game market with a registration system.Whilst downloadable titles – seemingly
intended as the main delivery method – will of course be at the whim of
Sony’s highly reliable servers and not subject to resale or unrestricted
transfer, Sony honcho Shuhei Yoshida has confirmed physical copies will be playable without restriction even if previously owned:

[Do you agree that if you buy something on a disc, that
you have a kind of moral contract with the person you've bought it from
that you retain some of that value and you can pass it on?]
“Yes. That’s the general expectation by consumers. They purchase
physical form, they want to use it everywhere, right? So that’s my
expectation.”
[So if someone buys a PlayStation 4 game, you're not going to stop them reselling it?]
“Used games can play on PS4.”

Another interview carried a similar confirmation by Yoshida, also seemingly ruling out region locks:

“When you purchase disc-based games for PS4, they will work on any hardware.”

Suspicions that Sony was planning on eliminating the second-hand
sales market as a way of reclaiming the billions in revenue it
apparently siphons away from game publishers and into the coffers of
game retailers flared after it emerged that Sony recently patented a
method of blocking used games from functioning, although Sony denies it
has anything to do with the PS4.
However, in other comments
Yoshida also revealed that publishers will be able to add anti-resale
registration schemes to their titles themselves, potentially allowing
them to block or discourage resale without any PR hassle for Sony.